Nef is known to enhance the pathogenicity of HIV-1 isolates. Deletions of nef have also been associated with attenuation of viral pathogenesis in both humans and primates. One feature of Nef's influence on pathogenicity is its ability to down-regulate CD4 and MHC on the cell surface. Another key aspect of Nef's modulation of pathogenicity is its ability to influence the intrinsic infectivity of viral particles. The increase in infectivity of Nef (+) viruses is mirrored by an increase in the production of proviral DNA. Deletions of nef result in a reduction in the efficiency of proviral DNA formation. While our understanding of CD4 and MHC down-regulation is fairly comprehensive, the mechanisms involved in the enhancement of viral infectivity are basically unknown. In this study we are interested in understanding how Nef is able to influence proviral DNA formation and virion infectivity. We are using a combination of biochemical and genetic approaches to help understand the mechanisms involved. In one approach, we are looking for specific interactions of Nef with other viral and cellular proteins to better understand the interactions going on at the molecular level. In another approach we are doing mutagenesis on the Nef protein to better understand the structure-function relationships of Nef. Since Nef plays a crucial role in the pathogenesis of HIV-1 a better understanding of the function of Nef will give us new and valuable insights into viral disease and potential therapies.